Given that the companies in the SAI 25 (or 47 in this case) don't share their numbers like a public company, the index is based on educated guesswork, looking at a combination of VC financing, financial performance, growth rate and market share and size.

Interestingly, Facebook, which was valued at $15 billion upon the $240 million investment by Microsoft, is valued at only $9 billion. That's still a lot of money for a company that did about $150 million in revenue last year. SAI includes dossier pages for each company in it index. It attributes the lower valuation of Facebook to the failure of the Beacon social advertising service to gain acceptance.

Even if the SAI index isn't accurate, it provides a framework for evaluating the mostly opaque startups and some fun for those who love tech company sports.